c. Electrophysiology 1
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- Why do we care about electrophysiology?
- Because ALL local and propagated electrical signals generated by a cell are driven by changes in the resting Em.
- Why is electrophysiology essential?
- It is necessary for maintaining cell, tissue, and organism homeostasis.
- What is "DIFFUSION POTENTIAL"?
- The ability of an ion to travel through a semipermeable membrane
- What 2 factors regulate the magnitude w/ which each permeable ion contributes to the resting membrane diffusion potential?
-
1. Transmembrane concentration difference of ea permeable ion
2. Mobility of permeable ion WITHIN the membrane relative to ions of opposite charge. - What is MOBILITY of an ion?
-
The velocity of ion movement
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Unit of Driving Force - What does difference in cation vs anion mobility have to do with Membrane Potential?
-
Leads to a:
-SEPARATION of charge
-Attractive force betwn ions
-ELECTRICAL POTENTIAL DIFFERENCE between the ions - What are the 2 determinants of ion intramembrane MOBILITY?
-
1. Diameter/shape of each ion and the pore thru which it goes
2. Ionic/Vanderwaals forces btwn ions and AA's in proteins facing the inner channel pore - Dipole
- An equal pos and neg charge separated by a finite difference
- What is Diffusion Potential a result of?
- Dipoles crossing the membrane in a nonrandom order due to force of a chemical gradient.
- Is the dffusion potential caused by a difference in charge between two components?
- NO - the intra/extracellular compartments are electroneutral actually; the separation of charge is ONLY IN THE MEMBRANE.
- What is the 'state' nature of the Diffusion Potential?
- STEADY STATE - requires a constant input of energy to maintain the chemical gradient.
- What is the equation for simple diffusion potential?
-
Pcat-Pan 2.3RT [IP]o
Ediff = -------- x ----- x ----
Pcat+Pan nF [IP]i - Units of Ediff
- millivolts mV
- What is the permeability of each ion proportional to?
- Its intramembrane diffusion coefficient (D)
- What is the Diffusion Coefficient (D) proportional to?
- The intramembrane mobility of an ion (beta)
- What else is permeability of an ion proportional to?
- the number of open channels in the membrane for each particular ion.
- What does 2.3RT/nF = ?
- 60 mV (at 37 ' C)
- When is the Nernst potential equation used instead of the simple diffusion potential equation?
- When one of the members of a cation/anion pair is IMPERMEABLE to the membrane.
- How is the nernst equationd different from that for simple diffusion potential?
- It is only for one ion (the cation), and so permeability is not a factor.
- What does the diffusion potential become when one member of a cation/ion pair is impermeable?
- Becomes the equilibrium potential for the permeable ion.
- What is an Equilibrium Potential?
- The condition where a CHEMICAL driving force is BALANCED by an EQUAL/OPPOSITE ELECTRICAL force.
- What is the equilibrium potential for potassium in a mammalian nerve or muscle fiber?
- -89 mV
- What is the normal K concentration inside/outside a mammalian nerve/muscle cell?
-
Inside = 140 mM
Outside = 4.7 mM - What happens to individual chemical/electrical potential differences when equilibrium potential is reached?
- The remain CONSTANT and DO NOT CHANGE.
- Is energy required to maintain the equilibrium potential for an ion?
- NO
- How much change in intracellular K would have to occur to set up an Ek of -88 mV (instead of -89 mV)?
-
For a squid axon membrane:
0.00001% - too small to detect. - What is the normal concentration of Ca2+ inside/outside a mammalian nerve or muscle fiber?
-
Ca inside = 10^-7 M
Ca outside = 1.6 x 10^-3 M - Based on the concentration gradient where does Ca want to go?
- INTO the cell - there is much more outside than inside.
- How neg/pos would a cell have to be to balance the concentration gradient and prevent Ca influx?
- VERY positive (to prevent the positive charge from rushing in)
- What would the Eca be if the membrane were to become permeable to it?
- 126.7 mV
- What equation is used for calculating the total resting membrane potential?
- GHK - goldman hodgkin katz
- What ions are included in the GHK equation for a squid axon?
- Na, K, Cl
- For a squid axon the measured resting Em is:
- -65 mV
- What is the effect of extracellular K on the resting Em in a squid axon?
- Increasing K outside will make the cell Em be closer to the Ek (hyperpolarize).